Condition responsive devices which provide an indication or control a selected operation in response thereto, are well known in the prior art. Such devices have been used in many diverse environments. For example, it is known in the prior art to connect a pressure sensitive element, such as a Bourdon tube, to a source of external pressure, whereby in operation the tube is deflected by an amount proportional to the sensed pressure variations. The mechanical deflection of the Bourdon tube is transduced into a mechanical output to provide an indication of the pressure in the tube or, alternatively, to control a physical operation. An apparatus of this type is shown in U.S. Pat. No. 4,502,334 and in U.S. Pat. No. 3,742,233, the latter describing a transducer for converting the motion of a Bourdon tube into a digital electrical signal.
Although such condition responsive devices have proven generally effective for indicating a condition or controlling an operation in response to a change in the condition, such devices may not be acceptable for providing the necessary degree of safety required in certain atmospheric conditions. For example, the use of the device such as described in U.S. Pat. No. 3,742,233 requires the installation of a light source at the pressure responsive element. This creates a condition for an electrically produced spark which in an explosive atmosphere would have disastrous results. In such applications where there is a potentially explosive environment, even a small electrically produced spark cannot be tolerated due to the potential of a disastrous explosion. Heretofore, compensation to minimize the potential of an explosion due to an electrically produced spark has required expensive and complicated instrumentation with its attendant high cost and potential unreliability.
Conventional C-shaped Bourdon tubes having one end closed are difficult to flush out and therefore have not been found acceptable for measurement of conditions in certain processes. The closed end construction is a disadvantage when it is desirable to measure conditions in a process wherein contaminants may impair the operation of the condition responsive element, such as the conventional C-shaped end Bourdon tube. As a result, Bourdon tube devices have not been widely utilized for measuring pressure and related phenomena where there is the potential for contaminating the Bourdon tube. The use of the closed end Bourdon tube has also been found objectionable in certain industrial processes wherein cleanliness is of paramount importance. The contaminants that build up in the closed end Bourdon tube have the potential for being flushed back into the process thereby contaminating the product. In the manufacture of semiconductor products for the electronics industry, this reverse contamination of the end product is of significant concern.
Accordingly, an advantage of the present invention is to provide a condition responsive apparatus having a flow through pressure sensitive element supporting the moving part of a transducer and operating in conjunction with a remote light source and a remote light detector coupled to the transducer by means of a fiber optic cable. The light detector inputs a signal to electronic circuits that employ a "look-up" table to characterize the apparatus output to follow the measured condition.